NO reduction by CH4 over well-structured Pt nanocrystal supported on γ-Al2O3

Article abstract of DOI:10.1246/cl.2001.1024

Platinum nanocrystals, having mainly (100) structure, have been prepared by a colloid method and then supported on γ-Al₂O₃. The catalytic test of Pt(100)/Al₂O₃ for the NO/CH₄ reaction showed that the formation of NH₃ and CO side products was prevented in contrast to the conventional Pt/γ-Al₂O₃ catalyst that gives relatively high selectivity to NH₃ and CO.

Full text of DOI:10.1246/cl.2001.1024

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Chemistry Letters 2001
NO Reduction by CH over Well-Structured Pt Nanocrystal Supported on γ-Al O
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Ioan Balint, * Akane Miyazaki, and Ken-ichi Aika††
†
††
†
Institute of Physical Chemistry, Romanian Academy, Spl. Independentei 202, 77208 Bucharest, Romania
††
Department of Environmental Chemistry and Engineering, Interdisciplinary Graduate School of Science and Technology,
Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8502
(Received July 11, 2001; CL-010645)
Platinum nanocrystals, having mainly (100) structure, have
been prepared by a colloid method and then supported on γ -
Al O . The catalytic test of Pt(100)/Al O for the NO/CH
2
3
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3
4
reaction showed that the formation of NH and CO side prod-
3
ucts was prevented in contrast to the conventional Pt/γ -Al O
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3
catalyst that gives relatively high selectivity to NH and CO.
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The use of CH as a reductant is an efficient way to remove
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nitrogen oxides. Platinum supported on alumina is a very active
catalyst for NO conversion but exhibits high selectivity to CO
and NH3.1 In a structural sensitive reaction, such as the conver-
2
sion of NO to N , the control of the morphology of the support-
2
ed Pt particles is essential to improve the catalytic activity and
the selectivity to the desired reaction products. A convenient
way to prepare monodispersed and well-defined metal particles is
the colloid method. An additional advantage is that the support
effect on the metallic active phase is minimized.3 Not only the
particle size but also the crystal structure of the metal nanoparti-
cles can be, to some extent, controlled by using appropriate struc-
4
ture-directing polymers.
From general point of view our investigation intend to
bridge the gap existing between the science of well-defined sin-
gle crystal surfaces under high vacuum condition and the word of
more practical interest of real catalysis. The particular aims of
our research are (I) to prevent the formation of undesired prod-
ucts (CO and NH ) during the catalytic reduction of NO with
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CH and (II) to get a better understanding of the relationship
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existing between the structure of supported Pt and the reaction
selectivity to various products. Our approach to the above-men-
tioned goals was to prepare well-defined Pt nanocrystals by using
the colloid method, to support them on alumina and to test the
catalytic activity and selectivity to various products for the
NO/CH reaction.
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The Pt nanoparticles were prepared by following the method
of Miyazaki et al.4 The water dissolved K PtCl (10 M) was
–4
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4
–3
reduced in the presence of a structure directing agent NIPA (10
M) (the polymer of N-isopropylacrylamide) with H at 40 °C for
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1
2 h. The resulted colloidal Pt nanocrystals were supported on γ -
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–1
alumina (Aerosil, 73 m g ) (the loading of Pt on alumina was
%). The catalytic test for NO/CH reaction was performed in a
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quartz microreactor, after freeze-drying and calcination (500 °C,
0 h) steps. The GHSV (Gas Hour Space Velocity) of the reactant
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–1
gas mixture (1% NO, 0.6% CH and balance Ar) was 60000 h .
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Typically, the colloidal Pt nanoparticles formed at 40 °C,
using as directing agent NIPA, were cubic in shape (around
Microscopy-Philips CM20), is a Gaussian having the maximum
located at around 12 nm. There was also a tiny fraction of very
small Pt nanoparticles whose size is centered at around 7 nm
(Figure 2). The TEM investigations showed that the morpholo-
gy of Pt nanoparticles did not change after deposition on alumi-
na (Pt nanoparticles retain their initial cubic shape).
70%) (Figure 1). Small amounts of round, hexagonal, triangu-
lar and irregular Pt crystals have been formed too.
The size distribution of the Pt nanoparticles, statistically
determined from the TEM micrographs (Transmission Electron
Copyright © 2001 The Chemical Society of Japan

Chemistry Letters 2001
1025
The catalytic activity of the Pt(100)/Al O for the reduction
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of NO with CH as a function of temperature is presented in
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Figure 3. The NO is completely converted (reduced by CH ) to
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N and N O starting from 400 °C. The selectivity of the reac-
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tion to N has a maximum at 500 °C (around 73%). The high-
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est selectivity to N O, decreasing with increasing temperature,
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was observed at 350 °C (around 64%). As interesting facts it
should be noted that, below 600 °C the formation of NH could
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not be observed and the only product of the methane oxidation
was CO (the formation of CO could not be detected at any
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reaction temperature).
In the case of well-structured Pt(100) nanoparticles sup-
ported on alumina the formation of CO as well as of NH is
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suppressed. On the other hand, selectivity to N O is higher
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than that observed for the classically prepared Pt/Al O (Figure
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3
4).
It seems that, on the surface of Pt(100) nanocrystals the
Oads species (from NO decomposition) rapidly react with CHx
species (derived from CH ) to give selectively CO and H O.
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2
2
The concentration of CH on Pt(100) is kept low by the high
x
activity of Oads and that is why formation of ammonia, via Nads
(
from NO) + H
(originating from CH ) reaction, is
ads 4
5
prevented. The higher selectivity to N O observed for support-
2
ed Pt(100) can be explained by the lower activity of Pt(100) to
dissociate NO as compared with other crystalline faces.
Therefore, the reaction between N and NOads to give N O is
The influence of the Pt particle morphology on the catalyt-
ads
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ic reduction of NO with CH is of great interest from a practical
favored.
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and theoretical point of view. Therefore, a comparison between
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the conventionally prepared Pt/Al O catalyst and our well-
A research fund from Japanese Society for the Promotion
of Science (No. P00136) is greatly appreciated.
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defined Pt(100) nanocrystals supported on alumina for the con-
version of NO and CH and for the selectivity to various prod-
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o
ucts at 500 C is presented in Figure 4. In this manner the
References
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1
2
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of the Pt particles on the reaction between NO and CH . The
4
conventional 1% Pt/Al O catalyst was prepared by incipient
2
3
1
wetness method from Pt(NO ) (NH ) precursor. The BET
surface area of Al O was 180 m g . The average metal parti-
3
2
3 2
2
–1
4
5
6
2
3
cle diameter of 1.8 nm was calculated from H chemisorption
2
data assuming a spherical geometry.